Smooth operators: lifting the lid on battery management systems
Just as the contribution of software updaters to better IT security can be taken for granted, the role of battery management systems (BMS’s) in the success of portable electronic devices is sometimes overlooked. Modern battery packs – found in notebook PCs and elsewhere – require accurate power monitoring circuits capable of determining the state-of-charge across multiple cells. Without this data, it’s impossible to ensure that cells are operating within safe limits. Peak current demands can be managed for short time periods, but will severely degrade the lifetime of the battery pack if they are allowed to persist.
ICs supplied by Texas Instruments and other chip makers, provide solutions for smartphones, laptops, and other battery-powered electronics. And the packaged hardware includes voltage regulators, temperature and current sensors, electrostatic discharge protection, chemical fuses, plus authentication and control systems. Roles for the BMS include checking the load being drawn on the power pack and reacting accordingly – for example, by issuing a request that connected circuits reduce their current demand if the battery drops too close to its low voltage limit. During charging, the BMS will bring about a reduction in current should the cell voltage climb too close to its upper limit. Again, to preserve a longer working life for the battery pack as a whole.
EV road trip
One of the most demanding environments for battery monitoring systems – but at the same time a use case where they can deliver tremendous value – is in electric vehicles. Unlike indoor scenarios such as the running of uninterrupted power supplies (another beneficiary of battery monitoring systems) – electric vehicles are driven in all weathers. Battery packs need to be resistant to the elements, but at the same time able to host cells within ideal operating windows – again, informed by BMS logic.
Events such as hard acceleration (which places a high current demand on cells) and braking-assisted power regeneration (used to return charge to the battery pack) need to be accommodated. And, ideally, vibration levels should be kept low, or damped out, to minimize stresses on the microstructured internals that facilitate the efficient flow of electrons. EV battery packs are complex, as patent submissions (such as this one from Tesla, which was granted in 2022) show. Cells may be thermally insulated from each other, and can feature cold plates or heat pipes as engineering solutions for temperature control.
More extreme still are the battery designs used in Formula E – an all-electric motorsport series that made its debut in 2014 as a showcase for sustainable mobility. The hi-tech power packs propel cars at speeds up 174 mph (280 kph) – for Gen2 designs – and demand clever engineering solutions to satisfy not just electrical needs, but also safety, aerodynamic, and weight requirements. Transportation events must be factored in too and battery packs are tested according to UN section 38.3 ‘Lithium and lithium ion batteries (PDF) requirements.
Designers have no shortage of issues to contend with, but a key one – as you would have gathered – is temperature management. For example, remember the Tesla patent highlighted earlier? A quick word search returns 56 and 28 hits for ‘heat’ and ‘thermal’, respectively. If the battery is allowed to run too hot then its lifetime will be reduced as degradation mechanisms accelerate at elevated temperatures. Conversely, cooler conditions don’t play so well with battery chemistry and affect the storage capacity of the pack. Regular readers may recall from a related article that, depending on the chemistry, some batteries may lose as much as a third of their charge when temperatures drop.
Formula E’s systems feature liquid cooling for efficient heat management, driven by inputs from the BMS. Liquid cooling adds complexity, but can dramatically improve power pack lifetime compared with units that feature passive designs. In fact, it’s a feature to look for if you find yourself shopping for an EV. And, to get you started, commercial electric vehicles that reportedly have liquid cooling include models from Tesla and BMW, as well as the Chevy Volt, Ford Focus Electric and Jaguar I-PACE, to give just a few examples.
Information gathered by the BMS can also help to flag when batteries should be swapped out for reuse – for example, by cascading units from road-going EVs into alternative applications. Since 2020, car maker BMW has been using end-of-life batteries with a high residual capacity in forklift trucks at plants in China. And once reuse has run its course, there’s recycling to consider too.
Materials firms such as Umicore – which has been involved in the recycling of race units from Formula E, as well as projects with BMW and Northvolt, a battery developer – point to the need for a ‘closed loop’ approach to lower the environmental footprint. There are financial drivers too, given the scramble to secure supply-constrained materials such as nickel, lithium and cobalt.
“In light of the growing scarcity of finite resources and rising commodity prices, it is especially important to push forward with the circular economy, said BMW’s Jochen Goller, commenting on the firm’s recycling activity. Battery power is booming, with e-mobility a big driving force – from scooters to luxury vehicles (BMW’s Rolls-Royce brand will be all-electric by the early 2030’s, according to the firm). And battery management systems play an important part in all of this – optimizing battery operations and more.
It’s no surprise to learn that BMS skills are in demand. Over 37,000 learners have signed up to Gregory Plett’s online course- ‘Introduction to battery-management systems’, available this month on Coursera, which covers a wide range of topics such as BMS sensing, high-voltage control, and other design requirements. Pletts is a Professor of Electrical and Computer Engineering at the University of Colorado at Colorado Springs, US, whose research on control systems ties in with the control of high-capacity battery systems deployed in hybrid and electric vehicles.
5 October 2022
4 October 2022